Complex shaped fiber for particle and molecular filtration

ABSTRACT

An ultra-efficient multilobal cross-sectioned fiber filter for chemical contaminant filtering applications is described. An absorptive chemically reactive reagent, preferably an acid or base and in liquid or an adsorptive chemically reactive reagent (an acid or base) in solid form, is disposed within longitudinal slots in each length of fiber. The reagent may be used alone or in conjunction with solid adsorptive particles which may also be utilized with the reagents in the longitudinal slots within the fibers. Reagents within the fibers remain exposed to a base-contaminated airstream passing through the filter. Base contaminants in the airstream, chemicals such as ammonium and amines (as well as particles), react with the acid reagent within the longitudinal slots of the fibers. As the contaminant and reagent react, the ammonium or amine becomes irreversibly absorbed (or adsorbed if reagent is a solid acid) to the liquid acid reagent and multilobal fiber.

[0001] Maintaining environments free of contaminants is particularlycritical in the manufacturing of integrated circuits because wafers arevery susceptible to small particles and low levels of certain chemicals.This can be done by manufacturing wafers inside cleanrooms with filteredair. The filters are used to reduce particle and chemical levels toextremely low levels (less than 1 part-per-billion). Semiconductor toolsare also sometimes equipped with environmental controls that providelocal ultra clean airflow during processing. However, conventionalchemical filters have a very short life span, require frequentreplacement, and are ineffective at efficiently filtering out certainchemicals.

SUMMARY

[0002] In accordance with the invention, an ultra-high efficientmultilobal fiber filter is described with long life use for chemicalcontaminant filtering applications. The unexpected ultra efficiency ofthe fiber filter reduces contaminants to low levels in theparts-per-billion. A reactive reagent, preferably an acid or base and ineither liquid or solid form, is disposed within longitudinal slots ineach length of fiber. The reagent may be reactive with base contaminantsby any known mechanism, such as an acid-base reaction to form ionicbonds, an oxidation-reduction reaction, and various other organic andinorganic reaction mechanisms as known in the art to form covalentbonds, hydrogen bonds, coordination compounds, or complex compounds. Thereagent may be used alone or in conjunction with solid adsorptiveparticles which may also be utilized with the reagents in thelongitudinal slots within the fibers. The fibers are formed into asingle layered, in one embodiment, or a multi-layered fiber mat, inanother embodiment, but the reagents remain exposed to the flow of acontaminated airstream passing through the filter. The contaminants in afluid stream react within the longitudinal slots of the fibers. As thebase contaminant and reagent react, the contaminant is retained withinthe longitudinal slots of the fiber.

[0003] In a preferred embodiment, the contaminants are ammonium and/oramines and the reagent is an acid. The acid-impregnated multilobal fiberis significantly more efficient than several commercially availablefilters. The filter can be used in a variety of applications includingclean rooms and in filtering chambers for installation in the field.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1 shows a detailed cross-sectional view of an individualmultilobal fiber.

[0005]FIG. 2 shows a close-up view of a fiber mat made of multilobalfibers with a reagent disposed within each multilobal fiber.

[0006]FIG. 3 shows a wider view of the fiber mat of FIG. 2.

[0007]FIG. 4 shows a plot comparing the percentage of contaminantbreakthrough over time for the acid-impregnated multilobal fiber versustwo other commercially available filters.

[0008]FIG. 5 shows a chamber in one embodiment for use in filtering anairstream with the acid-impregnated multilobal fiber filter.

[0009]FIG. 6 shows the chamber of FIG. 5 with a multi-layered multilobalfiber filter.

[0010] Use of the same reference symbols in different figures indicatessimilar or identical items.

DETAILED DESCRIPTION

[0011]FIG. 1 shows a detailed view of a cross section of an individualmultilobal fiber 2. Fiber 2 is a type of fiber made of thermoplasticpolymers and formed by conventional fiber forming techniques, such asspinning a fiber composition through a conventional spinnerate, asdescribed in the above-referenced U.S. Pat. No. 5,057,368. Fiber 2includes a core 4, from which three lobes 6 extend outwardly. Each oflobes 6 terminates with a cap 8 which is perpendicularly attached to theend of lobe 6. The cavity formed between lobe 6 and adjacent caps 8 runsalong the entire length of multilobal fiber 2 forming longitudinal slot10. Multilobal fiber 2 has the ability to retain powdered particulatematter, such as powdered carbon. The carbon powder can be mechanicallyheld within slot 10 entrapped by caps 8 without the use of any liquids.Entrapping the powder within slot 10 can be accomplished by dusting thepowder into the fibers and then shaking off the excess powder or blowingthe excess powder off with a fan.

[0012] Preferably, fiber 2 can hold, through capillary forces, liquidswithin slot 10. FIG. 2 shows a close-up view of fiber mat 100 whereliquid reagent 12 is filled within slots 10 of meshed fibers 2. Liquidreagent 12 can be utilized to fill slots 10 by having liquid dropletspenetrate fiber mat 100 and wicking into slots 10 after impacting withthe surfaces of fibers 2. After the liquid droplets impact with fibers2, they quickly coalesce into slots 10 while leaving open voids betweenfibers 2 and allowing for unencumbered airflow through fiber mat 100.Fibers 2 can alternatively wick reagent 12 up within slots 10 bycapillary force by dipping fibers 2 into reagent 12 and removing excessreagent 12.

[0013] Reagent 12 can be formed into liquid droplets by differentmethods such as forcing reagent 12 through a mechanical atomizer orpreferably by using a conventional liquid dropper. Reagent 12 can rangefrom a variety of liquids such as acids, oxidants, reductants,complexing agents, coordinating agents, and deliquescent agents;however, it is preferable to use acids. Acids for use as reagent 12include, but are not limited to, inorganic acids such as boric acid andpreferably phosphoric or sulfuric acid. Organic acids may also be usedrather than inorganic acids. Organic acids include, but are not limitedto, moncarboxylic, dicarboxylic, and tricarboxylic acids; these types oforganic acids include citric, lactic, maleic, fumaric, caproic, lauric,oxalic, malonic, tartaric, succinic, salicyclic, and malic acids. Inanother embodiment, powdered acids may also be used in place of theliquid acid and impregnated within slots 10 in a method similar to thatdescribed above for carbon powder. In an alternative embodiment,powdered bases may also be used to filter non-basic contaminants. Suchpowdered bases may include, e.g., sodium bicarbonate (baking soda),sodium carbonate, sodium hydroxide, trisodium phosphate, potassiumcarbonate, potassium hydroxide, and sodium tetraborate. Also, reagent 12may include coordinating agents which comprise transition metals, e.g.,copper, and complexing agents which comprise entrapping agents, e.g.,cyclodextrines.

[0014]FIG. 3 shows a wider view of fiber mat 100. Fibers 2 can bepleated or formed in layers to form fiber mat 100 in a variety ofconfigurations. Generally, the volume within slot 10 makes up about halfthe volume of multilobal fiber 2 and depending upon the density of thecontaminant, fibers 2 can gain around 100% in weight of liquidcontaminants and anywhere from 25% to 125% in weight for solid powderswithin slots 10. Utilizing the multilobal fiber 2 property to captureboth liquid or solid contaminants, reagent 12 is used for adsorptive andabsorptive filtration applications. Reagents 12 are intentionallyimpregnated within slots 10 and can be a reactive liquid or a solidreagent, preferably both acid, and fiber mat 100 is used as a supportivenetwork to hold reagents 12 in a highly dispersed configuration foradsorptive and absorptive molecular contaminant removal. Multilobalfiber 2 can retain not only a liquid or solid (acid) reagent 12, but iseffective in also retaining a combination of a liquid reagent and solidparticulates. Solid adsorbants such as zeolites, aluminum oxides,activated carbons (both impregnated and virgin), and chemically modifiedsilicas can be combined with acid reagent 12, in either liquid or solidform, and impregnated within slots 10 of multilobal fibers 2. Solidadsorbants may also be used in combination with base reagent 12 andimpregnated within slots 10.

[0015] Fiber mat 100 is particularly effective in filtering basecontaminants from a passing fluid stream. Bases are considered anychemicals or compounds conventionally regarded as a base in the chemicalarts. These chemicals and compounds include Lewis bases, shift bases,aqueous bases, and preferably any compounds that are alkaline in anaqueous environment. Ammonium and amines are preferable bases.

[0016] A fiber mat 100 impregnated with liquid acid reagents 12 isparticularly effective in specifically absorbing base contaminants, suchas ammonium, NH₃, and amines (for example, n-methyl-2-pyrrolidone (NMP))from a passing fluid stream. An airstream with contaminants, forexample, ammonium ions, passes through fiber mat 100. As it passes overmultilobal fibers 2 impregnated with acid reagent 12, the ammonium ionchemically reacts (in a reaction well known in the art) with and isadsorbed by acid reagent 12 to form a salt. Acid reagent 12 exchangeshydrogen atoms with the ammonium ions, forming water as a byproduct andleaving the ammonium ion to react with and attach to acid reagent 12.Reagent 12 irreversibly retains the ammonium ions within the slots 10 ofmultilobal fiber 2.

[0017] The combination of multilobal fibers 2 impregnated with a liquidacid reagent 12 gives unexpected, ultra-high efficient results infiltering out base contaminants. The effectiveness of fiber mat 100utilizing multilobal fibers 2 impregnated with acid reagent 12 can beseen in FIG. 4 when compared to two conventionally availableacid-impregnated commercial filters. All three filters were subjected toa continuous 90 parts-per-million (ppm) exposure of a gas contaminatedwith ammonium diluted in air. The air was conditioned to 50% relativehumidity and 23° C.; the air was blown through the filters at a velocityof 150 ft/min giving an equivalent pressure drop for all three filters.The basic gas concentration was measured continuously downstream of eachfilter and the breakthrough percentage of the contaminants were plottedover time. The breakthrough percentage is the amount of contaminantsremaining in the air downstream of the filter relative to the amount ofcontaminants initially in the air upstream of the filter.

[0018] As seen in FIG. 4, curve A represents multilobal fiber 2impregnated with acid reagent 12. Curves B and C represent the resultsof other commercially available filters. The results of curve A showsignificant retention of contaminants from the acid-impregnatedmultilobal filter 2 over either curve B or C showing the effectivenessand longer life of the acid-reagent 12 used in conjunction withmultilobal fiber 2. The retention time of fiber mat 100 is nearly threetimes as long as the retention time shown in curve B (120 minutes atabout 0% breakthrough versus 40 minutes at about 4% breakthrough forcurve B).

[0019] Fiber mat 100 utilizing acid reagent 12 in multilobal fiber 2 canbe used in a variety of applications, e.g., pleating or layeringmultilobal fiber 2 to form fiber mat 100. Fiber mat 100 can be used, inone embodiment, as a filtering element in a filtering chamber 102 asshown in FIG. 5. Chamber 102 consists of a housing 18, which can bedesigned from a conventional metal, e.g., aluminum, to accommodatehandling and field installation. The pleated or layered fiber mat 100 isattached to housing 18, preferably by an adhesive or a low outgassingglue. It is also possible, in an alternative embodiment, to have fibermat 100 attached to a frame (frame is not shown); this frame with fibermat 100 can then be installed or removed interchangeably within housing18. Housing 18 also has an inlet 14 and an outlet 16 to allow anairstream 20 to pass through filtering chamber 102 while being filteredthrough fiber mat 100. Chamber 102 can be used to provide particle andchemical filtration for ammoniums, amines, and particles for chemicalclean room environments.

[0020]FIG. 6 shows an alternative embodiment of fiber mat 100. FIG. 6 issimilar to chamber 102 in FIG. 5 in most respects except for thesubstitution of multi-layered fiber mat 100′ for fiber mat 100.Multi-layered fiber mat 100′ is a filter composed of a number ofindividual adjacent layers. Each individual layer is impregnated withany of the reagents discussed above; and several individual layers, eachlayer with a different reagent, are combined into a single multi-layeredfiber mat. The number of layers can range from one to N and fiber mat100′ can contain any combination of layers and reagents depending uponthe desired functionality.

[0021] Although the invention has been described with reference toparticular embodiments, the description is only an example of theinvention's application and should not be taken as a limitation. Inparticular, even though much of preceding discussion was aimed at liquidacid-impregnated multilobal fibers 2, alternative embodiments of thisinvention include multilobal fibers 2 impregnated with solid acidreagents 12 and multilobal fibers 2 impregnated with a base, both inliquid and solid form to filter non-basic contaminants. Various otheradaptations and combinations of features of the embodiments disclosedare within the scope of the invention as defined by the followingclaims.

We claim:
 52. A gas filter comprising: a plurality of elongated fibers,each fiber defining a plurality of longitudinally extending internalcavities; and a reactive reagent disposed within the internal cavitiesof the fibers, and selected from the group consisting of an acid, abase, a coordinating agent, a complexing agent, and a deliquescentagent.
 53. The filter of claim 1, wherein the reagent comprises an acid.54. The filter of claim 1, wherein the reagent comprises a base.
 55. Thefilter of claim 1, wherein the reactive reagent is impregnated in anadsorptive solid.
 56. The filter of claim 1, wherein the adsorptivesolid is selected from the group consisting of carbon powder, zeolite,aluminum oxide, and silica.
 57. The filter of claim 1 wherein each ofthe plurality of fibers is trilobal.
 58. The filter of claim 1 whereineach of the plurality of fibers is quadrilobal.
 59. The filter of claim1 wherein each of the plurality of fibers contains a plurality of Tshaped lobes.
 60. A method of filtering a gaseous contaminant from a gaswith a filter according to claim 1, comprising: selecting the reactivereagent to chemically react with the contaminant; impregnating thereactive reagent in an adsorptive solid; and disposing the adsorptivesolid in the plurality of internal cavities.
 61. The method of claim 60wherein the contaminant is acidic.
 62. The method of claim 60 whereinthe contaminant is basic.
 63. The method of claim 60 wherein thecontaminant is pH neutral.
 64. The method of claim 60, wherein theadsorptive solid is selected from the group consisting of carbon powder,zeolite, aluminum oxide, and silica.